Nuclear Magnetic Resonance Study on the Pnictide Superconductors in High Magnetic Fields.

摘要

NMR measurements in pnictide superconductors such as Ba(Fe 0.93Co0.07)2As2 (BaCo7), Ba0.67 K0.33Fe2As2 (BaK33), NaFe0.983 Co0.017As (NaCo17), and NaFe0.975Co0.025 As (NaCo25) over a wide range of magnetic fields are reported in this thesis. Studying different types of dopant and doping levels made it possible to find common properties and a hint for understanding relationships between spin density wave and superconductivity in the pnictide superconductors.;From 75As NMR in BaCo7 I found a spin-spin relaxation time, T2, changes due vortex dynamics in the superconducting state. Below the superconducting transition temperature, there is a dip in T 2 which is due to abrupt change in vortex dynamics and the temperature is consistent with the vortex freezing temperature as a function of the field. By changing an external magnetic field up to H = 28 T, the vortex phase diagram was obtained.;From 75As and 23Na NMR on NaCo25 a spin-singlet state consistent with weak coupling BCS theory was found. The diamagnetic magnetization was determined from 23Na NMR and subtracted to calculate the superconducting gap by fitting to the Yosida function. Additionally, the superconducting penetration depth was determined to be lambdaab (0) = 5,327 +/- 78 A based on Ginzburg-Landau theory.;The field dependent spin-lattice relaxation rate, 1/T1, was investigated on BaK33 and NaCo25 by 75As NMR up to H = 28 T. The two pnictide superconductors have an identical field dependence of 1/T1T at T = 4 K which is proportional to H2. The extended density of states from the vortex core is the most probable vortex related behavior which can explain the quadratic field dependence of 1/T1T.;The microscopic coexistence of a spin density wave (SDW) and superconductivity was found in NaCo17. From the 23Na and 75As NMR spectra it was shown that the SDW is incommensurate and the SDW is locally attenuated. The coexistence of an SDW and superconductivity in NaCo17 was conrmed from measurements of 1/23T1T and 1/ 75T1T.